Along with micropatterning of a semiconductor element, an exposure light source shifts from a high-pressure mercury lamp of a g-line to a light source of an i-line having a shorter wavelength, and further shifts to KrF and ArF excimer lasers having shorter wavelengths. Also, in order to increase resolution, the NA (numerical aperture) of a projection lens must be increased. Hence, the depth of focus tends to be reduced. As is known well, these relationships are expressed by:(resolution)=k·(λ/NA); and(depth of focus)=±k2·λ/NA2,where λ represents the wavelength of light used for exposure, NA represents the NA (numerical aperture) of the projection lens, and k1 and k2 represent coefficients for a process.
Recently, KrF and ArF excimer lasers with short wavelengths have been used. Also, an F2 excimer laser, an EUV light source, and an X-ray source are examined for use. Alternatively, the technique for increasing the resolution and the depth of focus by a phase-shift mask, modified illumination, or the like, is practically used. However, the cost of the apparatus increases when using the F2 excimer laser, EUV light source, and X-ray source.
An immersion technique has been attempted to be applied to the projection exposure apparatus, which uses the existing ArF excimer laser. Note that in this immersion method, the space between the projection lens and the wafer is filled with a liquid with a high refractive index to increase the resolution and the depth of focus. For example, the immersion-type projection exposure apparatus is disclosed in Japanese Patent Laid-Open No. 06-124873.
More specifically, in the immersion technique, as shown in FIG. 13, the space between a wafer 2 and an optical element 7, which is the last stage of the projection lens, is filled with a liquid 26. Assume that the wavelength of the exposure light in the air is λ0, the refractive index of the liquid 25 used for immersion with respect to the air is n, the convergent half-angle of a light ray is α, and NA0=sin α The resolution and the depth of focus obtained by using the immersion technique is expressed by:(resolution)=k1·(λ0/n)/NA0; and(depth of focus)=±k2·(λ0/n)/(NA0)2.That is, the immersion effect is equivalent to setting an exposure wavelength to 1/n. In other words, when designing projection optical systems with the same NA, the depth of focus can be n times by the immersion technique. This immersion technique can be applied to all pattern shapes, and used in combination with the phase-shift mask technique, the modified illumination technique, or the like.
In order to utilize this effect, the purity, uniformity, temperature, and the like, of the liquid must be accurately managed. In the exposure apparatus which sequentially exposes the wafer in a step-and-repeat operation, it is important to minimize the flow and oscillation of the liquid during the operation, and to eliminate air bubbles left on the wafer surface when loading the wafer in the liquid.
In a general projection exposure apparatus, which does not use the immersion technique, an illuminance unevenness sensor, which measures the illuminance and the illuminance unevenness of the exposure light irradiating the wafer, is generally disposed near the wafer. Also, a sensor, which measures the magnification and the focal position variation of the projection lens for calibration, is generally arranged near the wafer.
As a sensor unit for calibration, a light-receiving element is sometimes arranged under silica glass on which a reference mark pattern is formed by chromium, and the like, as shown in FIG. 4. Such a sensor unit is used, as will be described below. That is, a pattern similar to the reference mark in FIG. 4 is formed on a reticle. This pattern is projected on a reference mark, and the light-receiving element measures a light amount transmitted through the reference mark. When an in-focus state is set, the projection image of the pattern formed on the reticle matches the pattern of the reference mark. The light amount detected by the light-receiving element becomes maximum. In an out-of-focus state, the pattern formed on the reticle is blurred, and the blurred pattern is projected on the mark pattern. The light amount detected by the light-receiving element is reduced. Therefore, the focus varies while detecting the light amount by using the light-receiving element. The focus calibration can be performed by detecting the in-focus position where the light amount becomes maximum.
When the reference mark is displaced in the X and Y directions with respect to the pattern projection image on the reticle, the detected light amount changes accordingly. Therefore, the shifts in the X and Y directions can also be detected. That is, this sensor unit can be used as an alignment sensor unit. In addition to this, for example, a line sensor, which includes many pixels for measuring exposure unevenness in a shot and measuring an effective light source, and a two-dimensional CCD for measuring the aberration of the projection lens, may be disposed on a wafer stage.
In consideration of measurement precision, and the like, this sensor unit is preferably used in a state (immersion state) wherein the space under the projection lens is filled with the liquid as in the wafer, in the immersion-type projection exposure apparatus.
However, when using the above-described sensor unit in the immersion state, the following problems will occur.
FIG. 12 shows a conventional illuminance unevenness sensor unit. In the illuminance unevenness sensor unit, a light-receiving element 23 is arranged in a sensor vessel 22 sealed by a sealing window 21. A light-shielding member 28, which has a pinhole, is arranged on the sealing window 21. The light-receiving element 23 is arranged on the sealed space to prevent the characteristics of the light-receiving element 23 from degrading due to humidity, and the like. When the light is applied to the sensor unit having this structure, as is known by Snell's law, the light is refracted at an interface between the air and the sealing window 21, and an interface between the sealing window 21 and the internal space of the sensor unit.
Assume that the refractive index of the outer space of the sensor unit is n1, the refractive index of the sealing window 21 is n2, the refractive index of the internal space of the sensor unit is n3, the incident angle and refractive angle of the light at the interface between the outer space of the sensor unit and the sealing window 21 are θi1 and θr1, respectively, and the incident angle and the refractive angle of the light at the surface between the sealing window 21 and the internal space of the sensor unit are θi2 and θr2, respectively. The following equation is established by Snell's law:n2·sin(θi2)=n3·sin(θr2).Therefore,sin(θr2)=(n2/n3)·sin(θi2).When n2>n3, the light which is applied at an incident angle of θi2′ (critical angle) or more defined by sin(θi2′)=n3/n2 is not refracted, but totally reflected.
The NA of the projection lens is given by:NA=n1·sin(θi1).
In a general projection exposure apparatus which does not use the immersion technique, since the outer space of the sensor unit is filled with air, the refractive index n1=1. When the refractive index is 1, the NA is less than 1.0, or at most around 0.9. When the NA=0.83, the incident angle θi1=56.44°. When the material of the sealing window 21 is silica glass, the refractive index n2=1.56. By Snell's law, the refractive angle θr1 at the interface between the outer space of the sensor unit and the sealing window 21 is 32.29°. Since θr1=θi2, the incident angle θi2 of the light at the interface between the sealing window 21 and the internal space of the sensor unit is also 32.29°. The critical angle θi2′ at the interface between the sealing window and the internal space of the sensor unit is given by:sin θi2′=n3/n2=1/1.56=0.64.Therefore, since θi2′=39.9° and θi2′>θi2, the light is not totally reflected, but can reach the surface of the light-receiving element.
Alternatively, in the immersion-type projection exposure apparatus, the outer space of the sensor unit is filled with the liquid. Assume that the liquid is pure water. The refractive index of the pure water is 1.44. When the refractive index n1=1.44 in the above example, the incident angle θi1=56.44°, and the refractive index n2=1.56. Hence, by Snell's law, the refractive angle θr1 at the interface between the outer space of the sensor unit and the sealing window 21 is 50.28°. Since θr1=θi2, the incident angle θi2 of the light at the interface between the sealing window 21 and the internal space of the sensor unit is also 50.28°. As described above, the critical angle θi2′at the interface between the sealing window 21 and the internal space of the sensor unit is given by:sin θi2′=n3/n2=1/1.56=0.64.Therefore, since θi2′=39.9° and θi2′<θi2, the light is totally reflected. The light, which is applied at an incident angle of 39.9° to 50.28°, is totally reflected at the interface, and cannot reach the surface of the light-receiving element. Hence, the light amount cannot be accurately measured by using a total light beam, thus posing a problem. This problem affects not only the illuminance unevenness sensor unit, but also the above-described calibration sensor unit, or the like.
The discussion above is related to the problem in the immersion-type projection exposure apparatus. Alternatively, the projection exposure apparatus having a high NA has a factor which degrades the measurement precision of the conventional sensor unit which does not use the immersion technique. Generally, a photo-sensor has a characteristic to change the sensitivity in accordance with the incident angle of the light at the light-receiving surface. Hence, in the conventional projection exposure apparatus, the total light beam reaches the light-receiving element without total reflection. However, since light having a large incident angle at the light-receiving surface of the sensor is present, the light amount cannot be accurately measured upon reception of the influence of the angle characteristic of the sensitivity, thus posing a problem.
A complex optical system arranged on the sensor can cope with these problems. However, in consideration of the sensor unit disposed on the stage which moves at a high speed while holding the wafer, it is very difficult to implement the complex optical system in terms of weight and space.